Initial production of petroleum from subsurface reservoirs, referred to as primary production, frequently is accompanied by produced water (brine). The produced water must be separated from the oil and discarded or otherwise disposed of. Frequently the separated produced water is pressure injected into the same or another oil reservoir with the intention of increasing oil recovery from the reservoir. Such a reinjection process is referred to as a secondary recovery process with subsequent treatment processes referred to as tertiary processes. During primary oil recovery, the oil/water ratio in produced fluids is typically one part water to three parts of oil. During secondary and tertiary oil recovery, the water content in the produced fluids frequently increases to an oil/water ratio of fifty or more parts water to one part oil until continued production is no longer economically viable.
For water to be satisfactorily injected into a reservoir whether for purposes of disposal or improved oil recovery, the water should be sufficiently "clean" to enter the subsurface formation (a porous medium) without causing formation plugging. If the subsurface formation becomes plugged, progressively higher water injection pressures may be required to maintain a desired injection volume. At some point, the fracture pressure of the formation can be reached resulting in possible fracturing of the formation. If fracturing occurs the resulting "thief zone" in the fractured formation can establish low resistance paths through the formation that can steal most of the injection water. The result is a poor injection profile and the bypassing of formation oil which might have been captured in secondary or tertiary recovery. Furthermore, subsequent plugging of a "thief zone" is costly, difficult and often impossible.
Deposition of solids in the porous subsurface formation and on the face of the injection wellbore from injection waters may result in the need for an injection well "workover" or a stimulation of some kind within the formation. Processes such as mechanical scraping and bailing, acidizing, solvent washing and various combination treatments can be used but such processes are very expensive. In some cases an expensive side-track well may be drilled. In other instances, the original well can only be abandoned. A substantial economic incentive exists, therefore, to provide clean (solids-free) water for injection into the subsurface formation in a water injection process to avoid formation damage.
Water filtration equipment is affected by the cleanliness or the water separated from the produced fluids. Poor water clarity, whether due to inadequate filtration or the post-precipitation of solids, also causes severe problems in other surface facilities. In the surface facilities of oil field equipment, inorganic deposits (scale), coated by gelatinous solids, frequently called organic slime, often adhere to the internal surfaces of filters, pumps valves, storage tanks, and distribution pipelines. When these surface facilities become clogged the equipment must be shut down and cleaned. This introduces costly downtime and maintenance expenses. "Pigging" (mechanical internal reaming) of pipelines is sometimes required to maintain pipeline capacity.
Water quality is also affected by system upsets which occur with surges of oil flowing into the surface water treating equipment, seemingly at random periods, requiring frequent change of filter media and/or frequent filter backwashing cycles.
It is also known that oil well scale in the form of carbonate scale depositions is due to the loss of carbon dioxide from bicarbonate bearing waters. The petroleum industry has struggled with carbonate scale deposition and has generally attacked the problem by the addition of scale inhibiting chemicals, acidizing techniques, and various types of filters in series. None of these approaches have been directed toward the source of the problem; i.e., the loss of carbon dioxide from bicarbonate bearing waters.
Several recent changes have introduced new considerations to oil production and oil field water treatment in which carbonate-bicarbonate chemistry is involved. These changes include: